They figured they kill him and the project dies with him. Unfortunately that was not the case. Think Hydra killing Dr. Erskine to stop him from creating more super soldier serum.
They did not understand the situation correctly. In Rogue One, the Rebels still didn't have a solid and cohesive leadership.
They were stupid as in their Intelligence expert would have made Tarkin amused and that's NOT a compliment! All they needed was get him out, make sure Krennic had no idea Galen had contacted them and they might not have needed the Death Star plans... he was the one who installed it so with Tarkin & Krennic playing silly buggers it would have been easy to get aboard the Death Star and tinker with the exhaust system for the superlaser so when it was used it had no means to relieve the pressure causing it to blow up! Small unprotected exhaust vent, it needed several to be even practical! Sorry 38 years and I realise all that time and it only had ONE exhaust vent they could shoot at?!!!
They were going on bad/incomplete intelligence, and made a wrong call. It happens in war all the time.
I don't think this has been directly stated so I'll add something: I don't think they knew that the death star was complete. I recall when General Draven was informed that Jedha was destroyed and the look of shock on his face - he didn't know how that would happen or by what means. As previously stated the Rebels figured if they took him out the project would fail, or at least be stalled for some time.
Because he was a collaborator and a valuable asset to the enemy. Their primary concern was denying the enemy completion of their superweapon. Their intel did not know that the weapon was almost finished and that the delay up to that point had in fact been engineered by Erso. A capture must have also been considered an unacceptable risk to the alliance's security.
General Draven seemed to be sure that it's a good idea- he seemed like quite a hardcore rebel anyways but also he didn't really know enough. He seemed to be acting on his own mostly. I think his tactics would not be accepted by many other rebels since he was obviously very willing to engage in harsh tactics with questionable morality. Surely he would be darksider if force-sensitive
I think it was a personal choise by General Draven. He didn't think it was safe to extract Galen from an Imperial base, but he kept it unofficial, so there would be no problem from Mon Mothma nor Jyn. And I understand the choise, as Galen would be a real prime target, leading to massive operation, most likely the whole navy AND Deathstar going after the Alliance. Alliance would have become too big target if they captured the main designer of the Deathstar.
Recall Vader's interpretation of Admiral Ozzel's brilliant strategy to come out of lightspeed too close to the Hoth system and you will have your answer.
My question is why they did not try to kill more scientists? There were a whole team of engineers... anyone of them could have become the next Galen Erso... poor Galen just did it to himself by sending out the poor pilot, really.
Just finished this section in the novel Spoiler Draven learned of Krennic at the last minute. His first reaction was he'd hoped Cassian had time for a second shot to also take out Krennic. So there's an answer to the question of why Galen and not ________ for assassination.
The Rebels were in a bad position in Rogue One and for all we know the Emperor fed bad intelligence to the Alliance in order to sew discord among them. Saw was fighting his own war on Jedha.
I really disliked that guy. If you kill Imperial non-combatants in cold blood, without so much as a trial, you're no better than the Empire you're fighting. The means matter. If you're truly fighting to remove an evil government and you use their methods, you are hardly any better than the evil beings you're fighting to defeat. That's why the Rebellion needed good, honourable and just leaders like Bail Organa and Mon Mothma, who would not stand for such vicious tactics.
Draven was put in place by people like Bail and Mothma to make those cold decisions and manage the rebel army as needed. He felt taking out Erso would help stop the massacre of another million people. Based on what intel he had it was the best direction he could take. Taking out someone in high management causes all sorts of downstream problems - seems like a smart move imho.
But what about ethics? Morality? Justice, honor and decency? That is what the Rebel Alliance was built on. If you abandon those principles that founded your organization, you've forgotten why it is you fight in the first place. It's people like Draven and Saw, extremists and fanatics, that the Rebellion doesn't need pulling it down into immorality and ruthlessness. If Bail and Mon found out about Draven's order, I'm sure they would overrule it.
Mothma wanted to bring him to the Senate for testimony. Draven was a hardliner who didn't want to take any chances, and ordered him killed. A very realistic scenario.
Somebody help me - Draven is a new character who survives RO and doesn't show up in the OT, correct? Guess we'll get his story in a book at some point. He's not Snoke.
The whole mission was stupid. Cassian and Jyn original mission was to retrieve information from Saw Gerrera, get the renegade pilot if possible, and maybe convince Saw to change his methods and work with the Resistance... Jyn's presence was necessary to convince Saw Gerrera to collaborate, and they sent a single opperative with her (Cassian) so as to not provoke Saw Gerrera's paranoia... But once they had what they were sent to get, they should have been debriefed and a different team have been sent to rescue or kill Galen. Cassian didn't just have to carry out a difficult assessination on his own, he had to deal with Jyn, who would oppose him if she caught wind of what he intended to do (and would be a valuable hostage if captured) and drag around and protect the renegade pilot (who should have been interrogated for further information). How did General Draven expect the team to succeed?